1. Field of the Invention
The present invention relates to a ramp signal producing method for producing a ramp signal to drive a liquid crystal display, and to a ramp signal producing apparatus. The present invention also relates to a liquid crystal drive apparatus for driving a liquid crystal display in response to the ramp signal produced from the ramp signal producing apparatus, and to a liquid crystal display apparatus for displaying an image on the liquid crystal display.
2. Description of the Related Art
In a liquid crystal display apparatus, there are provided a liquid crystal display in which transparent electrodes are arranged in a matrix form to constitute pixels, an image signal driver for driving electrodes of the respective pixels of the liquid crystal display every horizontal line at image signal levels of the respective pixels, and a horizontal driver for driving the respective pixels of the liquid crystal display for each of the horizontal lines. The image is displayed on the respective pixels of the horizontal lines sequentially driven by the horizontal driver in response to the image signals for the respective pixels, whose levels are supplied from the image signal driver.
The horizontal driver drives the respective pixels by employing such a ramp signal having an asymmetrical trapezoidal shape, in which falling time is shorter than rising time. As a result, the respective pixels of the liquid crystal display are quickly discharged to erase the after images caused by the picture signal in the preceding frame. The extremely sharp rising edge as well as falling edge of this ramp signal are not produced in order to avoid deterioration of the liquid crystal display, or malfunction thereof during the drive operation.
The above-described ramp signal is produced by such a ramp signal producing apparatus constructed of an up/down counter for counting up a clock signal in the rising time and for counting down the clock signal in the falling time, and a ramp signal producing means for modulating the frequency of the clock signal multiplied by "m" during the falling time and for varying the amplitude of the clock signal in response to the count value of the up/down counter.
For instance, in the case that the count value is constructed of 7 bits and the amplitude of the ramp signal is variably set to 128 stages of analog voltage values by this ramp signal producing apparatus, when the D/A conversion time for the count signal is selected to be 17 .mu.s (microseconds), 128 pieces of burst clocks having the frequency of 7.5 MHz are required during the count up operation by the up/down counter. When the clock frequency of the clock signal is multiplied by four to produce the burst clock having the frequency of 30 MHz during the count down operation, the oscillation frequency of 60 MHz is required as the continuous clock.
Also, the white balance control and the offset value control are required so as to control the non-linear characteristic of luminance (brightness) based upon the non-linear characteristic of the transmittance characteristic of the liquid crystal display with respect to the applied voltage value. When the image signal level is represented by, for example, 256 gradation, the image signal level at the straight line portion may have resolution of 8 bits. However, the image signal level at the non-linear portion owns only resolution of 6 bits or less. Therefore, since the sensitivity of the half tone portion such as a grey portion becomes insufficient, this resolution of the non-linear portion must be increased four times higher than that of the straight line portion, namely resolution of 8 bits or more. Under such circumstances, the image signal level should own resolution of 10 bits.
On the other hand, when the clock frequency of the above-described ramp signal producing apparatus is selected to be 60 MHz, the logic circuits such as the up/down counter employed in this ramp signal producing apparatus must be fabricated in accordance with the very fine semiconductor manufacturing processes available for the high frequency operation. This may cause a high cost problem. Also, since the high frequency interference caused by the spurious radiation of the high frequency components contained in such a 60 MHz clock signal may occur in the VHF broadcasting band, even when the beat phenomenon would be suppressed by the shield member or the like, there is another problem that such high frequency interference cannot be completely eliminated. For instance, since 183.25 MHz corresponding to the video (picture) carrier frequency of the VHF 6 channel in the Japanese TV system is present around 180 MHz equal to the three-time higher harmonic of the 60 MHz clock frequency. Accordingly, the beat phenomenon should be seriously suppressed by the shielding means.
Also, in the case that the resolution of the image signal level is increased so as to control the white balance, the signal processing bit numbers of the A/D converter, the digital image signal processing circuit, and the ramp signal producing apparatus should be increased in order to improve the resolution. This A/D converter A/D-converts the image signal supplied to the liquid crystal display apparatus into the digital image signal. As a consequence, there is another problem that a large-scaled circuit arrangement is required and thus the manufacturing cost thereof would be increased.